The present invention relates to a method of forming an organic polymer insulating film in a semiconductor device.
The requirement for further shrinkage of feature size of semiconductor devices integrated in a semiconductor integrated circuit has been on the increase in accordance with semiconductor integrated circuit design rule. Such further scaling down of the semiconductor devices raises serious problems with resistance of interconnections and parasitic capacitance between the interconnections. Namely, a problem with delay in transmission of signals on interconnections in the semiconductor integrated circuit is raised. In order to solve the problem with the signal transmission delay, it is effective to reduce a dielectric constant of the inter-layer insulator between the interconnections. An organic insulating film has been known as a dielectric film having a low dielectric constant, for which reason the organic insulating film be attractive as the inter-layer insulator In prior art, the organic insulating film was formed by a spin coating method.
One of typical spin coating methods is disclosed in Mark R. Schneider, VLSI Multilevel Interconnect Conference June 1992, pp. 93-99, entitled "A Multilayer Interconnect Process For VLSI GaAs ICs Employing Polyimide Interlayer Dielectrics". This spin coating method will be described as follows.
In accordance with the spin coating method, a coating solution is first spin-coated on a base layer wherein the coating solution comprises a mixture of a source for growing a film with a solvent. Thereafter, a process for evaporation of the solvent is carried out. Subsequently, a curing process for the spin-coated film is carried out. An adhesion promoter may be, if necessary, coated on the base layer and subsequently baked, before the coating solution is spin-coated in order to ensure a good adhesion at an interface of the base layer.
Consequently, if the spin-coating method is used to form an organic insulating film, then at least three steps are necessary, for example, the spin-coating process for spin-coating the coating solution, the evaporation process for evaporating a solvent and the curing process. This spin-coating method, however, has the following four problems.
The first problem is concerned with many necessary processes and time-consuming method. As described above, at least three processes are necessary. Further, there may be added further processes for coating an adhesion promoter and subsequent backing thereof. Furthermore, the curing process requires a complicated temperature control. Namely, the necessary temperature profile of the curing process is complicated.
The second problem is concerned with a difficulty in accurate control to a thickness of the film. In accordance with the spin-coating method, the control to the thickness of the film is made by depending upon a viscosity of the coating solution and a rotation speed. An extremely high viscosity of the coating solution makes it difficult to form a thin film. By contrast, an extremely low viscosity of the coating solution makes it difficult to form a thick film.
The third problem is concerned with the use of a solvent to control the thickness of the film. The used solvent is required to be completely removed. Actually, however, it is difficult to remove the solvent completely and a small amount of the solvent is likely to remain in the film. In this case, the remaining solvent in the film may be evaporated, for example, in the post-process whereby voids and pinholes are formed in the film.
The fourth problem is concerned with generation of by-product in the process for forming an insulating film. This problem is serious particularly when polyimide is used. This problem is independent from the issue of whether or not the spin-coating method is carried out. During a reaction for forming imide rings, water is generated as by-product. Water or moisture remains in the film and may be evaporated from the inside of the film whereby voids and pinholes are formed in the film.
In the above circumstances, it had been required to develop a novel method of an organic insulating film free from the above first to fourth problems.